In the proper operation of a coke oven having one collector main along the coke side of the oven and another collector main along the pusher side of the oven, gas which is evolved from the coal-coke mass rises into the free space, i.e., the space between the top of the coal-coke mass and the roof of the coke oven, and flows in approximately equal quantities to each collector main. That is, about one-half of the gas quantity flows to the coke side collector main and the other half flows to the pusher side collector main. Such a flow pattern will be present if the pressures in the collector mains are about equal. If the pressures in the mains are slightly different, such a flow pattern may also be present because of an inherent resistance to flow described by the relationship: EQU .DELTA.P=8fLQ.sup.2 .rho./.pi..sup.2 g.sub.c D.sup.5 (Equation A)
where:
.DELTA.P=pressure differential between collector mains PA1 f=friction factor PA1 L=length of flow path PA1 Q=volumetric flow rate PA1 .rho.=density of gas PA1 g.sub.c =constant 32.2 ft. lb.m/lb.f sec..sup.2 PA1 D=diameter of flow path
If the pressure imbalance between the collector mains exceeds the .DELTA.P, inherent resistance to flow, crossflow can occur. As used herein, crossflow is defined as that condition which is present when gas from the higher pressure collector main flows down its standpipe, across the free space, up the other standpipe, and into the other collector main.
In recent times, the standpipe diameter and the free space height of coke ovens have been designed very large in order to prevent pressure buildup in the coke oven and thus reduce coke oven emissions. This increase in area available for gas flow has a profound effect on reducing the inherent resistance to flow because flow path diameter is raised to the fifth power in Equation A, above. For example, if the standpipe diameter is increased from 16 inches to 23 inches for a 45% increase in diameter, the resultant P in Equation A is decreased by 600%. Thus, the larger the flow areas become, the less pressure imbalance it takes between collector mains to cause crossflow. If the flow areas are large enough, the pressure imbalance to cause crossflow may be less than the pressure difference that can be controlled prior to this invention.
Crossflow is generally harmful to a coke oven because it disturbs the thermal conditions in the standpipe and free space, and can also lead to premature failure of the coke oven refractory material. In addition, crossflow can entrain flushing liquor, and the alkalies in the flushing liquor can enhance the growth of the silica brick adjacent the free space by accelerating crystallographic transformation and premature failure of the coke oven brick. Flushing liquor is a weak ammonia liquor generated in the coking process, collected external to the coke oven battery and pumped back through a nozzle to flush the internal surface of the gooseneck, i.e., the pipe that connects the standpipe to the collector main. Such flushing is done in order to prevent the accumulation of condensed tar on the internal surface of the gooseneck. The flushing liquor normally falls down into the collector main when gas flows from the standpipe through the gooseneck to the collector main, However, when crossflow occurs and the gas flows from the collector main through the gooseneck to the standpipe, the flow of gas entrains some flushing liquor and carries this liquor into the oven.
In most coke oven batteries it is desirable to completely eliminate crossflow. However, some coke oven battery designs inherently produce abnormally high temperatures in the roof of the coke ovens. Such high temperatures can result in excessive roof carbon formation. In such case, a controlled amount of crossflow may be desirable to reduce the amount of roof carbon formation.